you were only a child. Your parents were in debt and had to sell you to get some money to pay it off, not like they liked you a lot anyway.

After your new 'owner' got you he put you in a lab for human experimentarium along with a bunch of other kids, you got injected with all kinds of things and take a bunch of pills. But the heroes found out and raided the lab, but sadly most of the experiments were either dead or already relocated by the time they got there. But YOU, one of their few most resistance experiments were saved.

Keigo took his keys out to his apartment, unlocking the front door for you and him. The handcuffs on your wrist making sure you dont get into trouble. "Welcome to your new home for the time being, kid!" He said as he walked in while gently pulling you along by the cuffs.